Proper grounding for the best EMC design is still one of the most hotly debated topics of all time within the EMC and product design communities. The problem is that what worked for a past product or system may not work for another using more recent technology or clock speeds. Likewise, a list of “Rules of thumb” aren’t applicable for all cases and, by their very essence, divest the designer of their common sense and engineering judgment.
When it comes to shielded cables, do you ground one end or both ends? For PC boards, do you use separate analog and digital grounds. What about chassis grounds? What constitutes a “chassis” for those products that don’t have one? Do you use a single-point ground or a multipoint ground? What’s a designer to do?
These questions, and more, are answered in Elya Joffe’s and Kai-Sang Lock’s huge 1064-page, 3.5 pound, tome, Grounds For Grounding: A Circuit-to-System Handbook.
The authors try to provide a methodical approach to designing grounding systems, through circuits, systems, or facilities. According to Joffe, “From the topological perspective, there is no fundamental difference between a circuit, a rack, a platform, or a facility. The laws of electromagnetics revealed in Maxwell’s equations remain unchanged regardless of the system dimensions. Only the manner and complexity of their application differs from one to the next.”
Joffe continues, “We are confident that, eventually, this book will help do away with old, outdated, and erroneous practices, which may have been acceptable where low frequencies were concerned, but constitute poor design practices for the high-frequency circuits and systems so widespread today.”
Well, let’s dive in to the content and see if these claims are true.
The main content starts with chapter 2 and includes a lot of important basic electromagnetics theory, starting, of course, with a simplified explanation of Maxwell’s equations (Gauss’ Law for electric and magnetic fields, Faraday’s Law of u=induction, and Ampere’s Law, current in a wire produces a magnetic field, finishing up with boundary conditions.
But chapter 2 continues on with much more valuable basic information that provides proper “grounding” (sorry) for the concepts in later chapters. This would include the concept of inductance, self-inductance, mutual inductance, and the important concept of partial inductance, as well as internal and external inductance. The chapter finishes up with the high frequency behavior of components, return current path at low and high frequencies, transmission line fundamentals, spectral content of signals, differential and common mode signals, differential-to-common-mode conversion, differential mode and common mode radiation from conductors, and flux cancellation.
Once the basics are discussed, chapter 3 finally begins with “grounding”; the history, types, and objectives. The difference between safety, lightning, and grounding for EMI control is discussed next. The chapter concludes with several case studies.
Chapter 4 is a lengthy section on the fundamentals of grounding design. Common impedance coupling is described, along with ways to avoid it. Grounding topologies (single point, multipoint, and composite) methods are discussed. “Grounding trees” is an important visual tool that is described, along with a step-by-step procedure to create them. The importance of isolating and properly grounding switch-mode power supplies is discussed next.
The largest section in chapter 4 covers ground loops, why they’re formed, and how to avoid them. This is also one of the many misunderstood concepts in EMC design and the authors go into a lot of detail here. The chapter finishes up with a discussion of “zoned” grounding, that is, the concept of connecting multiple ground “zones” together in larger systems or racks.
Chapter 5 covers all aspects of bonding; gasketing, corrosion protection, and the mechanics of bonding metal structures together.
Chapter 6 covers grounding for power distribution (safety) for single and multi-phase systems and various means of lightning protection.
Chapter 7 discusses grounding in wiring circuits and cable shields. The theory of cable shields is covered in detail, as well as they methods of terminating cable shields. Cable shield quality and measurement of transfer impedance is discussed. The effect of pigtails and the degradation in shielding they cause is described. Grounding practices for ribbon cables is an important section, as well as proper grounding of signal interfaces and sensors, which completes this chapter.
Chapter 8 covers the design of filtering and transient protection of I/O ports.
Chapter 9 is the longest section in the book and covers all aspects of circuit board design, and between that, and the basics described in chapter 2, is worth the cost of the book. The authors start out with how signals propagate and the concept of return currents in power or ground return planes. Microstrip and stripline topologies are described, along with single-ended and differential transmission lines. Return currents at low and high frequencies are shown graphically. Other important topics include crosstalk and common impedance coupling.
Chapter 9 continues with the important concept of gaps in the return planes, a major cause of radiated emissions and immunity issues. Via design, fills, and anti-pads are discussed in relation to EMI practices. Then, daughter board connectors and appropriate ground return pins are discussed. One interesting concept now used in fast serial data lines is the idea of purposeful slots in the return planes below differential pairs. These “defected ground structures” (DGS). These DGS gaps in the return plane actually serve to reduce any common mode noise currents on signals running at microwave frequencies, such as Gigabit Ethernet and PCI Express II.
Chapter 9 goes on to describe simultaneous switching noise and the effective management of the EMI produced, noise and impedance control in power distribution networks, and decoupling principles. One major section covers a technique called “electromagnetic band gap” high impedance structures. This technique is used as a noise mitigation solution for parallel plate waveguides formed in conventional PC board stack-ups for microwave frequencies.
Finally, chapter 9 wraps up with PC board layer stack-up, image planes, local ground structures, copper fills, shield traces, via fences, guard rings, moating, circuit isolation, grounding in mixed signal boards, how to properly deal with A/D and D/A converters, properly connecting your circuit board to chassis, and grounding of heat sinks.
The final chapter 10 discusses grounding for facilities, ESD protection, grounding principles in mobiles systems, such as vehicles, aircraft, spacecraft, and ship board applications.
There is additional reference information, including a glossary of grounding terms and definitions, related acronyms, a list of symbols and constants, and a listing of grounding-related standards and handbooks.
In summary, Grounds For Grounding is much more than a book on “grounding”. It encompasses nearly a complete treatise on EMI control from a circuits and systems viewpoint. In addition, there are countless product design details of great value to the product or system designer.
While rather huge-looking, I’d recommend most product designers focus mainly on chapters 2, 3, 7, and 9. Parts of 4, 6, and 8 are useful, but chapters 2 and 9 are of most importance. Highly recommended!
Reference
Joffe and Lock, Grounds For Grounding: A Circuit To System Handbook, Wiley 2010, ISBN 978-0471-66008-8.
Amazon: https://www.amazon.com/Grounds-Grounding-Circuit-System-Handbook/dp/0471660086/
